The objective of this research is to study the mechanical events taking place in a normal cochlea at a cellular level. Increased understanding of the relationship between the basilar membrane vibrations and the effective mechanical stimulus acting upon the inner and outer hair cells should clarify the role of these two types of cell in the complex signal analysis that takes place in the cochlea. The techniques that have been utilized in the past for the basilar membrane vibration measurement have low sensitivity, low spatial resolution and introduce trauma and artifacts. A new measurement technique will be utilized to overcome these limitations. It will allow measurement of vibration of selected single cells in the normal organ of Corti. The measurement will be made interferometrically through an intact round window membrane at the basal end of the cochlea. Structures such as basilar membrane, tectorial membrane, inner and outer hair cells, pillar cells and supporting cells will be directly visualized and their vibrations determined in response to sound pressures colse to threshold of hearing. A wide variety of experiments are planned in order to determine frequency, phase response, linear and nonlinear properties. Since the resolution in direct viewing will be limited, the cells will be marked after measurement and subsequent histological analysis will allow us to directly relate the measured vibration amplitudes to the exact anatomical structures and locations in the organ of Corti. In another series of measurements, mechanical response of the cochlear structures will be determined before and after application of traumatizing stimuli. These experiments combined with a detailed histological study should help us determine the locus and the mechanisms by which cochlear injury occurs.